With the rapid progress of electronics, telecommunication and computer industries, demands for secondary batteries with high-performance and high-stability have been continuously increased in the art. In particular, in line with the increased consumption of compact, thin, light and portable electronic products, a secondary battery which is one of essential parts of the products, has been developed to meet the needs of small and light weight ones. In addition, as the number of automobiles is increased, the environmental destruction such as air or noise pollution as well as the petroleum exhaustion have been regarded as serious social problem, which drives the researchers to develop an alternative energy source and batteries having high generating power and high energy density.
Under the circumstances, a lithium polymer battery(“LPB”) has been proposed as one of the high-performance batteries for the next generation. LPB has a larger energy density per unit weight than that of conventional ones and can be processed in a diverse form, which eases the manufacture of high-voltage and large-capacity batteries by the technique of lamination. Furthermore, it does not employ any heavy metal such as cadmium or mercury causing environmental destruction, indicating that it is environment-friendly.
In general, LPB is composed of a negative electrode, a positive electrode and a polymer electrolyte, where the negative electrode includes lithium, carbon, etc., the positive electrode includes oxide of transition metal, metal chalcogen compound, conductive polymer, etc., and the polymer electrolyte comprises polymer, nonaqueous organic solvent(optionally), additives, etc., which has ion conductivity of about 10−3 to 10−8 S/cm at room temperature.
The polymer electrolyte, an essential component of LPB, is largely classified into a solvent-free polymer electrolyte and a plasticized polymer electrolyte. The solvent-free polymer electrolyte is composed of a polymer having polar groups and salts, where the polymer coats the salts and complex and ion is moved by the segment motion of polymer chain. On the other hand, the plasticized polymer electrolyte is composed of an excess amount of plasticizer, a polymer and salts, where the polymer plays a role as a supporter for the electrolyte and the salts are dissociated by the plasticizer to move ion.
Recently, extensive studies have been made on the economical and simple process for preparing a plasticized polymer electrolyte with a high ion conductivity at room temperature. However, the use of a polymer which plays a supporting role has proven to be less satisfactory in a sense that an excess amount of plasticizer is required to give a high ion conductivity, which brings about much difficulties in maintaining mechanical properties of the polymer. In this regard, a polymer electrode having a cross-linked structure has been proposed in the art. The plasticized polymer electrolyte with cross-linked structure has also revealed a shortcoming that its surface roughness is lager than that of a linear polymer electrolyte, indicating that its interfacial resistance with an electrode would be larger than that of linear polymer electrolyte and its interface characteristics under the electrical stress would be more unstable. In fact, Y. Aihara et al. reported that the plasticized polymer electrolyte was produced by plasticizing a random copolymer of ethylene oxide and propylene oxide with ethylene carbonate and propylene carbonate, whose interfacial resistance was measured about 1,000 to 1500Ω (see: Aihara Y., J. Power Source, 65:143, 1997).
Under the circumstances, there are strong reasons for exploring and developing an improved process for preparing LPB with lowered interfacial resistance between a plasticized polymer electrolyte and electrodes.